1. Field of the Invention
The invention relates generally to the field of image processing and computer graphics. More specifically, the invention relates to architectures and methods for performing image processing and compression.
2. Description of the Related Art
In small or portable devices such as digital cameras, the goal of image compression is to reduce the storage and processing requirements of the data while still maintaining acceptable picture quality. When storage and processing requirements are reduced, the overall power consumption of the camera is also reduced since the VLSI (Very Large Scale Integration) chip performing the processing is more compact. The reduction of the bit rate for transmission or storage of still image and motion video will also speed the process of capturing images and then, downloading them to a PC (personal computer) or other more complex data processing system.
Image compression techniques whether performed by hardware such as VLSI or otherwise by software can be classified as either xe2x80x9clossyxe2x80x9d or xe2x80x9closslessxe2x80x9d. With lossless compression, the original image prior to compression can be retrieved exactly when the compressed image is decompressed. Consequently, lossless techniques, whose compression ratios depend upon the entropy of an image, do not achieve high compression ratios and, since they preserve a high percentage of original image information, are computationally expensive. By contrast, lossy compression provides only an approximation of the original image. Thus, with lossy compression, greater compression ratios can be achieved but with loss in image quality compared to lossless techniques. One such lossy technique referred to as xe2x80x9cpredictive codingxe2x80x9d (also called Digital Pulse Code Modulation (DPCM) which is well-known in the art) predicts the value of a successive pixel by linearly combining the properties of already processed neighboring pixels. An error pixel is defined as the difference between the original image pixel and the corresponding predicted pixel. The error pixel, is quantized and then binary encoded. Traditionally, the quantization has been performed distinct from the encoding, which lends to complexity in the processing circuitry or software algorithm.
The implementation of such computationally intensive techniques demands more VLSI circuitry than is suitable for digital cameras and portable, small devices desiring image compression. Thus, there is a need for simpler architecture to perform these techniques while conserving power and preserving the accuracy of the compression technique.
An image compression apparatus is disclosed that includes a first look-up table providing a quantized code, a second look-up table providing the length of that code, and a predictive coding circuit which generates an address for indexed those look-up tables.